Flip-Chip Packaging Structure for Light Emitting Diode and Method Thereof

ABSTRACT

A packaging structure and method for a light emitting diode is provided. The present invention uses flip-chip and eutectic bonding technology to attach a LED to a thermal and electrical conducting substrate. The flip-chip packaging structure comprises a thermal and electrical conducting substrate having an insulating layer formed in an appropriate area on the top surface of the substrate and a bonding pad formed on top of the insulating layer; and a LED reversed in a flip-chip style and joined to the substrate by eutectic bonding. A first electrode of the LED is eutectically bonded to an appropriate area on the top surface of the substrate via a eutectic layer, while a second electrode of the LED is electrically connected to the bonding pad.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to packaging structures forlight emitting diodes, and more particularly to a flip-chip packagingstructure and a related method for light emitting diodes with enhancedheat dissipating efficiency.

2. The Prior Arts

Light emitting diodes (LEDs) have advantages of small size, low powerconsumption, long operation life, and therefore will substitute lightbulbs or fluorescent lamps in the future as the mainstream light source.However, to enhance the brightness of LEDs, it is necessary to increasepower, numbers, or density of LEDs. These will inevitably increase theheat produced by the LEDs. This heat, if not effectively dissipated,will damage the LEDs themselves, shorten their operation life, andaffect their brightness.

To improve heat dissipation, a packaging structure as shown in FIG. 1has been adopted. As illustrated, LED 10 is attached to an aluminumsubstrate 20 by eutectic bonding with a eutectic layer 30 interposedtherebetween. An insulating layer 21 is formed in specific areas on topof the aluminum substrate 20. Bonding pads 22 are in turn formed on topof the insulating layer 21, which are electrically connected to thefirst and second electrodes 11 and 12 of the LED 10 respectively viabonding wires 23. In this conventional approach, even though theeutectic layer 30 is able to dissipate the heat produced by the LED 10through the aluminum substrate 20 by conduction, the sapphire substrate13 of the LED 10 has inferior thermal conductivity and the heatdissipation efficiency is therefore impaired. Furthermore, the packagingstructure described above has the first electrode 11 and the secondelectrode 12 both formed on the top of LED 10 which blocks light emittedfrom LED 10 and decreases its brightness.

To overcome the foregoing problems, a flip-chip packaging structure asshown in FIG. 2 is adopted, where LED 10 is reversed and attached to aprinted circuit board (PCB) 40. The PCB 40 has bonding pads 41 formed onspecific locations on the top surface, and the bonding pads 41 areelectrically connected to bonding wires 42. The first and secondelectrodes 11 and 12 of the LED 10 are connected separately to bondingpads 41 using metallic bumps 43. Although the packaging structure indeedavoid blocking light emitted from LED, heat generated by the LED 10 canonly be conducted to the PCB 40 through the small contact area with themetallic bumps 43. Additionally, the PCB 40 itself has limited thermalconductivity and cannot provide satisfactory help to the heatdissipation.

SUMMARY OF THE INVENTION

In order to increase LED brightness and heat dissipating efficiencysimultaneously, the present invention provides a flip-chip packagingstructure and a related method which use eutectic bonding to attach aLED to a thermal conducting substrate. The flip-chip packaging structureand the method of the present invention avoid blocking the light emittedfrom the LED, and rely on a larger contact area and higher thermalconductivity to conduct heat to a thermal conducting substrate forefficient heat dissipation.

The flip-chip packaging structure of the present invention comprises athermal and electrical conducting substrate having an insulating layerformed on a specific area on the top surface and a bonding pad formed ontop of the insulating layer surface; and a LED revered in a flip-chipstyle and attached to the substrate. A first electrode of the LED isjoined to the substrate by eutectic bonding via a eutectic layer formedtherebetween. The second electrode, on the other hand, is joined to thebonding pad. The eutectic layer is formed by plating a metallic layer ontop of the t substrate corresponding to the first electrode prior to theeutectic bonding process. In an alternative embodiment, the eutecticlayer can also by formed by interposing a metallic plate between thefirst electrode and the corresponding location on the top surface of thesubstrate before conducting the eutectic process. The substrate can bemade of Al, Cu, AlN, or other appropriate material having high thermalconductivity.

In contrast to the prior arts, the LED packaged by the present inventiondoes not have its light blocked by the electrodes and therefore hasbetter brightness. The heat produced by the LED can also be conductedmore efficiently via the eutectic layer which has a much larger contactarea between the thermal conducting substrate and the LED, instead of,as in the prior arts, going through the sapphire substrate of the LEDwhich has inferior thermal conductivity. Therefore, the heat generatedby the LED can be conducted and dissipated to the thermal conductingsubstrate more effectively and efficiently.

The foregoing and other objects, features, aspects and advantages of thepresent invention will become better understood from a careful readingof a detailed description provided herein below with appropriatereference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view showing a conventional packaging structure.

FIG. 2 is a schematic view showing a conventional flip-chip packagingstructure.

FIG. 3 is a schematic view showing the flip-chip packaging structureaccording to an embodiment of the present invention.

FIG. 4 is a schematic view showing the formation of the eutectic layeraccording to an embodiment of the present invention.

FIG. 5 is a schematic view showing the formation of the eutectic layeraccording to another embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

With reference to the drawings and in particular to FIG. 3, FIG. 3 is aschematic view according to an embodiment of the present invention. Asillustrated, the flip-chip packaging structure comprises a LED 50 and athermal conducting substrate 60, bonded together by a flip-chiptechnology.

LED 50 comprises a first electrode 51 and a second electrode 52, wherethe first and second electrodes 51 and 52 are provided on a same side ofthe LED 50. The first and second electrodes 51 and 52 are made of metalslike Ti, Al, or Au formed by physical vapor deposition (PVD) and fusing.For a GaN-based LED die, the first electrode 51 can be joined with theP⁻ GaN layer and functions as the p-type electrode, and the secondelectrode 52 can be joined with the n⁺ GaN layer and functions as then-type electrode. Please note that this arrangement of electrodes isexemplary only and there are various other alternatives. Also, duringthe vapor deposition process, the first electrode 51 can be configuredto have a larger area and, together with an appropriate thickness, amore stable bonding and a better thermal conductive interface betweenthe LED 50 and the substrate 60 can be achieved during the subsequenteutectic bonding process. Then, once an appropriate voltage is appliedon the first and second electrodes 51 and 52, the LED 50 is turned on.

The thermal conducting substrate 60 can be made of Al (thermalconductivity coefficient 231 W/m·K), Cu (thermal conductivitycoefficient 385 W/m·K), or AIN (thermal conductivity coefficient 320W/m·K). Any other similar material could also be used but those havethermal conductivity coefficient higher than 100 W/m·K at roomtemperature are preferable. Besides providing heat dissipation, thethermal conducting substrate 60 of the present invention should alsohave superior electrical conductivity. Therefore, Al and Cu are the mostpromising candidates for the thermal conducting substrate 60. Beforeattaching LED 50 to the thermal conducting substrate 60, an insulatinglayer 61 is formed in a specific area corresponding to the secondelectrode 52 on the top surface of the thermal conducting substrate 60.The insulating layer 61 can be formed by chemical vapor deposition (CVD)using, but not limited to, SiO₂ or Si₃N₄. Then, a bonding pad 62 made ofan appropriate metallic material is formed on the top surface of theinsulating layer 61 for connecting to the second electrode 52. As willbe explained later, the first electrode 51 is electrically connected viaa eutectic layer 63 to the thermal conducting substrate 60 which is alsoelectrically conductive, there is no need to prepare a second bondingpad or have a bonding wire connected to the first electrode 51 fromexternal circuitry. As to the second electrode 52, its connection to theexternal circuitry can be achieved by a bonding wire connected to thebonding pad 62.

Then the LED 50 is reversed in the flip-chip style to attach on thethermal conducting substrate 60 using eutectic bonding. The secondelectrode 52 is in direct contact with the bonding pad 62, and aeutectic layer 63 is formed between the first electrode 51 and the topsurface of the thermal conducting substrate 60. As such, the largeamount of heat generated by the LED 50 can be quickly conducted throughthe first electrode 51 and eutectic layer 63 to the thermal conductingsubstrate 60 and dissipated. In contrast to the conventional approach,most of the light of the LED 50 is radiated out unblocked through thesapphire substrate 53. Most importantly, the heat is conducted throughthe eutectic layer 63 having a much larger contact area than that of themetallic bumps and a much better thermal conductivity than that of thesapphire substrate 53.

With reference to the drawings and in particular to FIG. 4 and FIG. 5,FIG. 4 and FIG. 5 are schematic views showing the formation of theeutectic layer 30 according to two embodiments of the present invention.As shown in FIG. 4, a metallic layer 631 made of appropriate materialssuch as Au and/or Al is platted on the top surface of the thermalconducting substrate 60 corresponding to the first electrode 51, andthen the eutectic layer 63 is formed by a eutectic bonding process atappropriate temperature. In an alternative embodiment, as shown in FIG.5, a metallic plate 632 made of appropriate materials such as Au and/orAl can be positioned between the first electrode 51 and the top surfaceof the thermal conducting substrate 60. Then the eutectic bondingprocess is conducted at an appropriate temperature. When the firstelectrode 51 is made of Cu, the foregoing processes would form a Cu/Auor Cu/Au/Al eutectic layer 63. Please note that the metallic layer 631and the metallic plate 632 can be made of other appropriate metallicmaterial and those with higher thermal conductivity coefficient arepreferable.

Although the present invention has been described with reference to thepreferred embodiment thereof, it is apparent to those skilled in the artthat a variety of modifications and changes may be made withoutdeparting from the scope of the present invention which is intended tobe defined by the appended claims.

1. A packaging structure for LEDs, comprising: a thermal and electricalconducting substrate; an insulating layer in an appropriate area on thetop surface of said substrate; a bonding pad on top of said insulatinglayer; an eutectic layer in an appropriate area not overlapping saidinsulating layer on the top surface of said substrate; and a LED havinga first electrode and a second electrode on a top side of said LED, saidLED being reversed so that said first and second electrodes facing thestop surface of said substrate, said first electrode being joined tosaid eutectic layer, and said second electrode being joined so saidbonding pad.
 2. The packaging structure as claimed in claim 1, whereinsaid thermal conducting substrate made of Al.
 3. The packaging structureas claimed in claim 1, wherein said thermal conducting substrate is madeof Cu.
 4. The packaging structure as claimed in claim 1, wherein saidinsulating layer is made of SiO₂.
 5. The packaging structure as claimedin claim 1, wherein said eutectic layer is eutectic Cu/Au.
 6. Thepackaging structure as claimed in claim 1, wherein said eutectic layeris eutectic Cu/Au/Al.
 7. A packaging method of a LED having a firstelectrode and a second electrode on a top side of said LED, comprisingthe steps of: (1) providing a thermal and electrical conductingsubstrate; (2) forming an insulating layer in an appropriate area on thetop surface of said substrate, and forming a bonding pad on top of saidinsulating layer; (3) reversing said LED so that said first electrodeand said second electrode face the top surface of said substrate; and(4) eutectic bonding said LED to said substrate so that a eutectic layeris formed and bonded between said first electrode and an appropriatearea not overlapping said insulating layer on the top surface of saidsubstrate, and said second electrode electrically connected to saidbonding pad.
 8. The packaging method as claimed in claim 7, wherein saidthermal conducting substrate made of Al.
 9. The packaging method asclaimed in claim 7, wherein said thermal conducting substrate is made ofCu.
 10. The packaging method as claimed in claim 7, wherein saidinsulating layer is made of SiO₂.
 11. The package method as claimed inclaim 7, wherein said step (4) further comprising: plating a metalliclayer in said appropriate area not overlapping said insulating layerbefore conducting said eutectic bonding.
 12. The packaging method asclaimed in claim 11, wherein said eutectic layer is eutectic Cu/Au. 13.The packaging method as claimed in claim 11, wherein said eutectic layeris eutectic Cu/Au/Al.
 14. The packaging method as claimed in claim 7,wherein said step (4) further comprising: interposing a metallic platebetween said appropriate area not overlapping said insulating layer andsaid first electrode before conducting said eutectic bonding.
 15. Thepackaging method as claimed in claim 14, wherein said eutectic layer iseutectic Cu/Au.
 16. The packaging method as claimed in claim 14, whereinsaid eutectic layer is eutectic Cu/Au/Al.